Cleaning in membrane filtration systems

ABSTRACT

A method of cleaning permeable, hollow membranes ( 6 ) in an arrangement of the type wherein a pressure differential is applied across the walls ( 15 ) of the permeable, hollow membranes ( 6 ) immersed in a liquid suspension, the liquid suspension being applied to the outer surface of the porous hollow membranes ( 6 ) to induce and sustain filtration through the membrane walls ( 15 ) wherein some of the liquid suspension passes through the walls ( 15 ) of the membranes ( 6 ) to be drawn off as clarified liquid or permeate from the hollow membrane lumens ( 7 ), and at least some of the solids are retained on or in the hollow membranes ( 6 ) or otherwise as suspended solids within the liquid surrounding the membranes ( 6 ). The method of cleaning comprises the steps of applying a cleaning solution ( 14 ) to one side of the membrane wall ( 15 ); applying a pressure differential across the membrane wall ( 15 ) to cause flow of the cleaning solution ( 14 ) through the wall ( 15 ) from the one side of the membrane wall ( 15 ) to the other side of the membrane wall ( 15 ) and applying a reverse pressure differential across the membrane wall ( 15 ) to cause flow of the cleaning solution ( 14 ) through the wall ( 15 ) from the other side of the membrane wall ( 15 ) back to the one side of the membrane wall ( 15 ). A method of determining the amount of chemical cleaning solution required is also disclosed.

FIELD OF THE INVENTION

The present invention relates to the backwashing of hollow permeablemembranes used in membrane filtration systems and, in particular, to animproved method of backwashing and cleaning the hollow permeablemembranes.

BACKGROUND ART

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

Known backwash systems include those described in our earlierInternational Application No. WO93/02779 the subject matter of which isincorporated herein by cross-reference.

During cleaning of membranes, cleaning solutions are often flowedthrough the membranes and their permeable walls to clean foulants fromthe membranes. Applying the cleaning solution under pressure assists theremoval of foulants from the surface.

The typical known cleaning procedure for membranes involves cleaning themembranes in-situ. This procedure adds a set concentration of chemical,commonly 2% citric acid followed by 200-1000 ppm sodium hypochlorite, tothe membrane in a solution of filtrate. This usually occurs at the startof the two hour cleaning period, after which the cleaning solution isfiltered through the membrane and left to soak.

As the nature of membrane fouling varies according to feed quality andtype, flux through the membrane and hours of operation, the amount andlength of chemical cleaning required in each situation also varies. Thisoften results in a one process fits all approach where a standardchemical cleaning stage is applied regardless of the amount of fouling.This can lead to large amounts of cleaning solution being usedunnecessarily with the effect of additional cost and environmentalimpact in disposing of the waste solution after cleaning is complete.

DISCLOSURE OF THE INVENTION

It is an object of the invention to overcome or at least ameliorate oneor more of the disadvantages of the prior art or at least provide auseful alternative.

According to a first aspect, the present invention provides a method ofcleaning permeable, hollow membranes in an arrangement of the typewherein a pressure differential is applied across the walls of thepermeable, hollow membranes immersed in a liquid suspension, said liquidsuspension being applied to the outer surface of the porous hollowmembranes to induce and sustain filtration through the membrane wallswherein:

(a) some of the liquid suspension passes through the walls of themembranes to be drawn off as clarified liquid or permeate from thehollow membrane lumens, and

(b) at least some of the solids are retained on or in the hollowmembranes or otherwise as suspended solids within the liquid surroundingthe membranes, the method of cleaning comprising the steps of;

-   -   i) removing, at least partially, liquid from the filtrate side        of the membrane;    -   ii) removing, at least partially, liquid from the outer side of        the membrane;    -   iii) applying a cleaning solution to the outer side of the        membrane;    -   iv) applying a pressure differential across said membrane wall        to cause flow of said cleaning solution through said wall from        the outer side of the membrane into the membrane lumen to at        least partially fill said lumen with cleaning solution;    -   v) isolating the outer side of the membrane;    -   vi) applying a pressurized gas to the filtrate side of the        membrane wall to cause flow of the cleaning solution back to the        outer side through the membrane wall;    -   vii) accumulating the increased pressure developed on the outer        side of the membrane as a result of said flow of cleaning        solution;

viii) releasing the pressure applied by said pressurised gas to saidfiltrate side of the membrane wall to cause flow of said cleaningsolution through said membrane from the outer side to the filtrate sideunder the effect of said accumulated pressure on the outer side of themembrane wall.

For preference the cleaning solution is a chemical cleaning solution.

Preferably, in step vi) gas, usually air, is applied such that themembrane lumen is substantially drained of cleaning solution. Preferablythe pressure is accumulated in step vii) in a gas space provided on theouter side of the membrane wall or alternatively in a bladderarrangement.

The differential pressure of step iv) may be provided by applying gaspressure to the outer side of the membrane wall or by applying a vacuumto the filtrate side.

According to a further aspect the present invention provides a method ofcleaning permeable, hollow membranes in an arrangement of the typewherein a pressure differential is applied across the walls of thepermeable, hollow membranes immersed in a liquid suspension, said liquidsuspension being applied to the outer surface of the porous hollowmembranes to induce and sustain filtration through the membrane wallswherein:

(a) some of the liquid suspension passes through the walls of themembranes to be drawn off as clarified liquid or permeate from thehollow membrane lumens, and

(b) at least some of the solids are retained on or in the hollowmembranes or otherwise as suspended solids within the liquid surroundingthe membranes, the method of cleaning comprising the steps of;

-   -   i) applying a cleaning solution to one side of the membrane        wall;    -   ii) applying a pressure differential across said membrane wall        to cause flow of said cleaning solution through said wall from        said one side of the membrane wall to the other side of the        membrane wall;    -   iii) applying a reverse pressure differential across said        membrane wall to cause flow of said cleaning solution through        said wall from said other side of the membrane wall back to said        one side of the membrane wall.

According to yet a further aspect the present invention provides amethod of cleaning permeable, hollow membranes in an arrangement of thetype wherein a pressure differential is applied across the walls of thepermeable, hollow membranes immersed in a liquid suspension, said liquidsuspension being applied to the outer surface of the porous hollowmembranes to induce and sustain filtration through the membrane wallswherein:

(a) some of the liquid suspension passes through the walls of themembranes to be drawn off as clarified liquid or permeate from thehollow membrane lumens, and

(b) at least some of the solids are retained on or in the hollowmembranes or otherwise as suspended solids within the liquid surroundingthe membranes, the method of cleaning comprising the steps of;

-   -   i) removing, at least partially, liquid from the filtrate side        of the membrane;    -   ii) removing, at least partially, liquid from the outer side of        the membrane;    -   iii) applying a cleaning solution to the outer side of the        membrane;    -   iv) applying a pressure differential across said membrane wall        to cause flow of said cleaning solution through said wall from        the outer side of the membrane into the membrane lumen to at        least partially fill said lumen with cleaning solution;    -   v) applying a pressure differential across said membrane wall to        cause flow of said cleaning solution through said wall from the        lumen side of the membrane back to the outer side of the        membrane lumen.

Preferably the pressure differential in step v) is produced by applyinga pressurized gas to the filtrate side of the membrane wall to causeflow of the cleaning solution back to the outer side through themembrane wall.

The cleaning process can be repeated in cycles such that the cleaningsolution is alternately moved from one side of the membrane to the otherthrough the membrane wall.

The process can be applied to membranes submerged in an open vessel aswell as pressurized membrane filtration systems.

According to another aspect of the present invention there is provided amethod of controlling a chemical clean of a membrane comprising:

measuring pH and/or membrane resistance of a membrane for at least aportion of said clean; and

ceasing said chemical clean when pH and/or membrane resistance attains apredetermined value.

According to another aspect of the present invention there is provided amethod of controlling a chemical clean of a membrane comprising:

measuring pH and/or membrane resistance of a membrane for at least aportion of said clean;

measuring elapsed time of the clean;

calculating a rate of change of pH with respect to time (dpH/dt) and/ora rate of change of membrane resistance (dR/dt) with respect to time;and

ceasing said chemical clean when dpH/dt and/or dR/dt attains apredetermined value.

According to another aspect of the present invention there is provided amethod of controlling the chemical cleaning of a filtration systemcomprising the steps of measuring membrane resistance of a membrane forat least a portion of said clean;

measuring elapsed time of the clean;

calculating a rate of change of membrane resistance (dR/dt) with respectto time; and

using dR/dt to calculate a duration for completion of the clean.

According to another aspect of the present invention there is provided amethod of controlling a chemical clean of a membrane comprising:

increasing the amount of chemical cleaning agent present during theclean;

measuring membrane resistance of a membrane for at least a portion ofsaid clean;

ceasing the increase in chemical cleaning agent when membrane resistanceattains a predetermined value.

Preferably, the amount of cleaning agent is increased incrementally.

For preference, the predetermined value approximates a steady-statevalue of membrane resistance. Preferably, the membranes aremicrofiltration or ultrafiltration type membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 a shows a simplified sectional side elevation of a membranemodule with a lower portion of the module immersed in a chemicalcleaning solution and suction applied to the membrane lumens;

FIG. 1 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 1 a;

FIG. 1 c shows an enlarged sectional view of the membranes in theindicated region of FIG. 1 a;

FIG. 2 a shows a simplified sectional side elevation of a membranemodule of FIG. 1 with a lower portion of the module immersed in achemical cleaning solution and pressurized gas applied to the membranelumens;

FIG. 2 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 2 a;

FIG. 3 a shows a simplified sectional side elevation of a membranemodule of FIG. 1 with a lower portion of the module immersed in achemical cleaning solution and suction applied to the membrane lumens;

FIG. 3 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 3 a;

FIG. 3 c shows an enlarged sectional view of the membranes in theindicated region of FIG. 3 a;

FIG. 4 a shows a simplified sectional side elevation of anotherembodiment of a membrane module with a lower portion of the moduleimmersed in a chemical cleaning solution and suction applied to themembrane lumens;

FIG. 4 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 4 a;

FIG. 4 c shows an enlarged sectional view of the membranes in theindicated region of FIG. 4 a;

FIG. 5 a shows a simplified sectional side elevation of the membranemodule of FIG. 4 with a lower portion of the module immersed in achemical cleaning solution and pressurized gas applied to the membranelumens;

FIG. 5 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 5 a;

FIG. 6 a shows a simplified sectional side elevation of anotherembodiment of a membrane module with a lower portion of the moduleimmersed in a chemical cleaning solution and suction applied to themembrane lumens;

FIG. 6 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 6 a;

FIG. 6 c shows an enlarged sectional view of the membranes in theindicated region of FIG. 6 a;

FIG. 7 a shows a simplified sectional side elevation of the membranemodule of FIG. 6 with a lower portion of the module immersed in achemical cleaning solution and pressurized gas applied to the membranelumens;

FIG. 7 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 7 a;

FIG. 8 a shows a simplified sectional side elevation of an embodiment ofa membrane module in an open vessel with a lower portion of the moduleimmersed in a chemical cleaning solution and suction applied to themembrane lumens;

FIG. 8 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 8 a;

FIG. 8 c shows an enlarged sectional view of the membranes in theindicated region of FIG. 8 a;

FIG. 9 a shows a simplified sectional side elevation of a membranemodule of the embodiment of FIG. 8 with a lower portion of the moduleimmersed in a chemical cleaning solution and pressurized gas applied tothe membrane lumens;

FIG. 9 b shows an enlarged sectional view of the membranes in theindicated region of FIG. 9 a;

FIG. 10 shows a graph of transmembrane pressure (TMP) measured over timefor a membrane module of the type illustrated in FIGS. 8 and 9undergoing a chemical clean using the method according to the invention;

FIG. 11 shows a graph of membrane resistance measured over time for twotypes of chemical cleaning process with incremental increases in thevolume of chemical cleaning agent added during the cleaning process; and

FIG. 12 shows a graph of membrane resistance measured over time withincremental increases in the volume of chemical cleaning agent addedduring the cleaning process.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the FIGS. 1 to 7, there is shown a membrane module 5 havinga plurality of hollow fibre membranes 6. The fibre membranes 6 havetheir lumens 7 closed at the lower end in a lower pot 8 and open at theupper end through upper pot 9. The module 5 is contained in a vessel 10having a controllable valve 11 for opening/closing the vessel 10 toatmosphere. Upper pot 8 is connected to a filtrate collection chamber 12having a port 13.

One embodiment of the cleaning process according to the invention willnow be described with reference to FIGS. 1 to 3. As best shown in FIGS.1 a to 1 c, liquid remaining in lumens 7 after filtration is drainedwhile liquid remaining in the vessel 10 is also at least partiallydrained. The vessel 10 is then, at least partially, filled with cleaningsolution 14 as best shown in FIG. 1 a. A differential pressure is thenapplied across the membrane walls 15, in this embodiment by applying avacuum to port 13, so that the cleaning solution 14 is drawn through themembrane walls 15 (as shown in FIG. 1 b) and up the membrane lumen 7until it is, at least partially, filled with cleaning solution.

As shown in FIGS. 2 a and 2 b, the valve 11 is then closed to isolatethe vessel 10 while pressurized gas is applied through port 13 to thecleaning solution filling the membrane lumen 7 and displaced through themembrane wall 15 until the lumen 7 is substantially drained of cleaningsolution. With the vessel 10 isolated, the transfer of cleaning solution14 through the membrane wall 15 to the outer side of the membrane 6results in the pressure within the vessel to increase as the gas space16 within the vessel 10 is compressed.

As shown in FIGS. 3 a to 3 c, the lumen side of the membranes are ventedto atmosphere. The accumulated pressure in the gas space 16 then forcesthe cleaning solution 14 to flow back through the membrane wall 15 asbest shown in FIG. 3 b.

FIGS. 4 a to 7 b illustrate embodiments of the invention where a bladderarrangement may be used instead of the gas space 16 to accumulatepressure.

Referring to FIGS. 4 a to 4 c, the operation is similar to that shown inFIG. 3 a to 3 c, however, in this embodiment when the lumen side of themembranes is vented to atmosphere through valve 17, the bladder 16delivers pressure to the feed side of the vessel 10 forcing the cleaningsolution 14 through the membrane wall 15 and along the membrane lumen 7as best shown in FIGS. 4 b and 4 c.

Referring to FIGS. 5 a and 5 b, the pressurising of the lumen/filtrateside is shown. Pressurised gas is applied to the lumen/filtrate side ofthe membranes 6 by feeding pressurised gas through line 18 and valve 17to port 13. The pressurised gas causes the cleaning solution within thelumen 7 to flow through the membrane wall 15 to the outer side of themembrane resulting in the pressure within the vessel 10 increasing andbeing transferred to the bladder 16 connected to the vessel 10 throughline 19 and valve 11.

FIGS. 6 a to 6 c show a similar arrangement to FIGS. 4 a to 4 c but inthis embodiment the gas pressure is applied to the vessel 10 from anexternal source, rather than the bladder 16, through line 19 and valve11. Bladder 16 is used to accumulate pressure on the lumen side of themembranes 6 as shown in FIG. 6 a.

As shown in FIGS. 7 a and 7 b, when the reverse flow of cleaningsolution is required, the vessel 10 is vented to atmosphere through line19 and valve 11 and bladder 16 releases accumulated pressure to thelumen side forcing the cleaning solution 14 within the lumens backthrough the lumen walls 15 (see FIG. 7 b).

Referring to FIGS. 8 and 9, an embodiment of the cleaning processaccording to the invention is illustrated where the vessel 10 is open toatmosphere. In this embodiment flow of cleaning solution through themembrane wall 15 is provided by alternately applying suction/vacuum orpressure to the lumen side of the membranes 6. The membrane module 5 isagain immersed at least partially in chemical cleaning solution 14 andsuction is applied to the open ends of the fibre membrane lumens 7. Asbest shown in FIG. 8 b, the cleaning solution 14 is drawn through themembrane wall 15 and into the fibre membrane lumen 7. The cleaningsolution 14 is then drawn up through the lumen 7 until it is completelyfilled as shown in FIG. 8 c. As shown in FIGS. 9 a and 9 b, pressurizedgas is then applied to the cleaning solution 14 filling the membranelumen 7 and the cleaning solution is displaced through the membrane wall15 as previously described.

The process illustrated in the embodiments can be repeated in cyclessuch that cleaning solution is alternatively moved from one side of themembrane wall 15 to the other. This flow of cleaning solution to andfrom the membrane lumens 7 and well as along their length results in aneffective chemical clean of the membrane module 5.

FIG. 10 shows the results of applying the cleaning regime according tothe invention to a membrane module of the type where the vessel 10 isopen to atmosphere. The cleaning process was performed as follows:

1. The membrane vessel was filled with filtrate via backfilling from thelumen side to the shell side, with simultaneous chlorine dosing into thefiltrate line. The vessel filtrate level was about 30%, with a targetvolume of cleaning solution (NaOCl) of 30 mL.

2. The filtrate was then recirculated briefly through the system inorder to ensure a well-mixed cleaning solution.

3. The lumens were then drained of liquid by 100 kPa air being appliedto the filtrate line. This allowed the cleaning solution to diffusethrough the pores and down the fibre length, which raised the filtratevessel level. This step may be ended when the liquid level stops rising.

4. The lumens were then filled with the cleaning solution by usingvacuum air applied to the lumen side of the membranes. During this stepthe level in the filtrate tank dropped as the liquid was pulled into thefibre lumens. This step may be ended when the liquid level stopsfalling.

5. The lumen fill and drain steps were repeated until contact time hadreached 1800 seconds.

6. After 1800 seconds of cleaning solution contact, the vessel wastopped up with feed. This allowed the remaining free chlorine in thecleaning solution to contact with the part of the module that wasexposed during the clean.

7. The system was then aerated to maximise contact of solution withmodule.

8. The tank was then drained and flushed with filtrate before returningto service.

The data graphed in FIG. 10 shows a period when the cleaning regime wasperformed once every 24 filtration hours for 4 days in succession, withthe module operating at 1.7 m3/hr with a 30 minute backwash interval.Chlorine in the form of sodium hypochlorite (NaOCl) was used, theaverage free chlorine concentration during the clean being 100 ppm. Feedwater turbidity was between 60-90 NTU throughout.

The data shown in FIG. 10 illustrates the regular reduction intransmembrane pressure (TMP) flowing each clean.

One embodiment of the invention seeks to minimise the amount of chemicalrequired by adding it incrementally to the membrane tank, whilstmonitoring resistance through the membrane during a recirculation stagein the cleaning process. Chemical additions can cease when the furtheraddition of chemical leads to change in the membrane resistance below apredetermined level, hence minimising the amount of excess chemicalagent used in the cleaning process.

The resistance value can be monitored during filtrate recirculation.Typically, during a standard cleaning procedure, the chemical cleaningsolution is recirculated at the start of the clean only, followed by upto 48 hours of soaking of the membranes. In the present embodiment, thechemical cleaning solution is recirculated for several minutes (forexample −3 minutes) every 15-30 minutes during the soak/aeration steps,allowing the membrane resistance to be measured periodically throughoutthe cleaning process.

When the change in resistance per 3 recirculations drops below apredetermined value (for example −0.1) the cleaning process hasrecovered the maximum performance at that chemical concentration andfurther chemical agent is added. When the addition of further chemicalagent effects the change in resistance by less than the predeterminedvalue per 3 recirculations (for example −0.1), no further recovery canbe achieved and the cleaning process can therefore be terminatedimmediately. Conversely, the cleaning potential can be maximized byextending the cleaning process so that the change in resistance per 3circulations is below a certain predetermined value. FIG. 11 shows agraph of resistance value variation of the duration of the cleaningprocess for two different cleaning regimes using citric (CIP1) andchlorine (CIP2) cleaning agents.

Referring to the graph shown in FIG. 12, the volume of chemical agentbegins around 100 ml as shown at A resulted in a significant drop inmembrane resistance. The amount of chemical agent was further increasedas shown at B and C resulting in further decreases in membraneresistance. Once the volume of chemical agent reached about 250 ml, themembrane resistance change reached substantially a steady state as shownat D and further increases (E) in chemical agent had minimal effect. Atthis stage the volume of chemical agent added can be ceased withoutadversely affecting the cleaning process and recovery in transmembraneflow.

It will be appreciated that using the above measurements it is possibleto determine a resistance profile during the cleaning process for aparticular membrane arrangement or configuration. The resistance profilecan then be used to predict the end of cleaning process time, half-lifeand reduce chemical use in simultaneous cleans of similar systems. Theresistance profile may be further used to determine whether chemicalsare required to be added during the cleaning process with the type andamount of chemical being dependent on feed and foulant quality.

Typical cleaning solutions which may be used include acids, causticsolutions and oxidizing solutions (e.g. chlorine).

The invention may be embodied in a similar apparatus to that describedin the aforementioned International Application No. WO93/02779appropriately modified to operate in accordance with the inventivemethod.

It will be appreciated that further embodiments and exemplifications ofthe invention are possible without departing from the spirit or scope ofthe invention described.

1. A method of cleaning a permeable, hollow membrane, the permeable,hollow membrane comprising a wall, a filtrate side, and an outer side,the method comprising: removing liquid from the filtrate side of themembrane; removing liquid from the outer side of the membrane; applyinga chemical cleaning solution to the outer side of the membrane; causingflow of the chemical cleaning solution through the wall from the outerside of the membrane into the filtrate side to at least partially fillthe filtrate side with the chemical cleaning solution by applying apressure differential across the membrane wall; isolating the outer sideof the membrane; causing flow of the chemical cleaning solution back tothe outer side through the membrane wall by applying a pressurized gasto the filtrate side; accumulating the increased pressure developed onthe outer side of the membrane as a result of the flow of the chemicalcleaning solution; and causing flow of the chemical cleaning solutionthough the membrane wall from the outer side to the filtrate side underthe effect of the accumulated pressure on the outer side of the membranewall by releasing the pressure applied by the pressurized gas to thefiltrate side of the membrane wall.
 2. (canceled)
 3. The methodaccording to claim 1, wherein applying the pressurized gas to thefiltrate side comprises applying gas such that the filtrate side issubstantially drained of the chemical cleaning solution.
 4. The methodof claim 1, wherein accumulating the increased pressure comprisesaccumulating the increased pressure in a gas space provided on the outerside of the membrane wall.
 5. The method of claim 1, whereinaccumulating the increased pressure comprises accumulating the increasedpressure in a bladder arrangement.
 6. The method of claim 1, whereinapplying the pressure differential comprises applying gas pressure tothe outer side of the membrane wall.
 7. The method of claim 1, whereinapplying the pressure differential comprises applying a vacuum to thefiltrate side.
 8. A method of cleaning a permeable, hollow membrane, thepermeable, hollow membrane comprising a wall, a one side, and anotherside, the method comprising: applying a chemical cleaning solutioncomprising at least one of an acid, a caustic solution, and an oxidizingsolution to the one side of the membrane wall; causing flow of thechemical cleaning solution though the wall from the one side of themembrane wall to the other side of the membrane wall by applying apressure differential across the membrane wall; and causing flow of thechemical cleaning solution through the wall from the other side of themembrane wall back to the one side of the membrane wall by applying areverse pressure differential across the membrane wall.
 9. The method ofclaim 8, wherein applying the pressure differential comprises applyinggas pressure to the one side of the membrane wall.
 10. The method ofclaim 8, wherein applying the pressure differential comprises applying avacuum to the other side of the membrane wall.
 11. The method of claim8, wherein applying the reverse pressure differential comprises applyinggas pressure to the other side of the membrane wall.
 12. The method ofclaim 8, wherein applying the reverse pressure differential comprisesapplying a vacuum to the one side of the membrane wall.
 13. (canceled)14. A method of cleaning a permeable, hollow membrane, the permeable,hollow membrane comprising a wall, a filtrate side, and an outer side,the method comprising: removing liquid from the filtrate side of themembrane; removing liquid from the outer side of the membrane; applyinga chemical cleaning solution comprising at least one of an acid, acaustic solution, and an oxidizing solution to the outer side of themembrane; causing flow of the chemical cleaning solution though the wallfrom the outer side of the membrane into the filtrate side of themembrane to at least partially fill the filtrate side with the chemicalcleaning solution by applying a pressure differential across themembrane wall; and causing flow of the chemical cleaning solutionthrough the wall from the filtrate side of the membrane back to theouter side of the filtrate side by applying a reverse pressuredifferential across the membrane wall.
 15. The method of claim 14,wherein applying the reverse pressure differential comprises applying apressurized gas to the filtrate side of the membrane wall.
 16. Themethod of claim 14, wherein the steps of the cleaning method arerepeated in cycles such that the chemical cleaning solution isalternately moved from a first side of the membrane to a second side ofthe membrane through the membrane wall. 17-24. (canceled)
 25. The methodof claim 1, wherein the chemical cleaning solution comprises anoxidizing solution.
 26. The method of claim 25, wherein the chemicalcleaning solution comprises chlorine.
 27. The method of claim 8, whereinthe chemical cleaning solution comprises citric acid.
 28. The method ofclaim 8, wherein the chemical cleaning solution comprises chlorine. 29.The method of claim 14, wherein the chemical cleaning solution comprisescitric acid.
 30. The method of claim 14, wherein the chemical cleaningsolution comprises chlorine.